The present invention, in some embodiments thereof, relates to chemical syntheses and, more particularly, but not exclusively, to novel processes of olefin polymerization, to catalyst systems which comprise, as pre-catalysts, metal complexes of group 4 Salan ligands which can be utilized in these processes, and to Salan ligand precursors for preparing these complexes.
The huge plastics industry produces a broad variety of polymeric materials having a broad range of properties. These plastic materials are derived from a small group of building blocks—monomers—including ethylene and propylene. The properties of the polymeric materials depend on the nature of these building blocks and on the process employed to assemble these building block. Most of these processes rely on catalytic polymerization.
The nature of the catalyst has a crucial role in determining the microstructure of the polymer thus determining the physical properties of the resulting plastic. The microstructural characteristics of polymers and copolymers of propylene include: molecular weight, molecular weight distribution (Mw/Mn; or PDI), and above all, the type and degree of stereoregularity (tacticity) and regioregularity (head-to-tail enchainment). These microstructural characteristics influence the properties of the resulting polymer. For example, three familiar forms of polypropylene are: isotactic in which all methyl side groups are pointing in the same direction in the stretched chain; syndiotactic in which the methyl side groups point at opposite directions alternatingly; and atactic in which the methyl groups are pointing randomly in the two directions. A higher degree of stereoregularity (and regioregularity) tends to lead to higher crystallinity and melting points. In particular, polypropylene having a very high degree of isotacticity has a melting transition of Tm=165° C. whereas an atactic polypropylene, if it has a melting point, is typically much lower.
The type and degree of tacticity are also influenced by the catalyst systems employed. Other properties influenced by the catalyst system include the polymer chain-lengths and chain-length distributions, possible backbone rearrangement, regio-regularity, ability to incorporate different monomers, etc. Successful catalysts also need to be sufficiently active under industrially-relevant conditions.
Many of the industrial catalytic processes employed in ethylene and propylene polymerizations and copolymerizations rely on heterogeneous catalysis processes, and most of which, on heterogeneous Ziegler-Natta type catalysts. Ziegler-Natta catalysts are group 4 metal complexes activated with alkyl-aluminum co-catalysts, which were invented in the 1950's. Recent generations of such catalysts include titanium chloride adsorbed on magnesium chloride and have various combinations of internal and external donors. More recent Ziegler-Natta catalysts are very active and can produce highly isotactic polypropylene (having a melting point of about 165° C.). Yet, these catalysts result in polymers having a broad molecular weight distribution (PDI=Mw/Mn>3.5), and their activity towards higher olefins is considerably lower.
The next generation catalysts, both homogeneous and heterogeneous, to be developed, are referred to as metallocenes. These catalysts are based upon transition metals (such as group 4 metals) having at least one group 4 cyclopentadienyl-type (Cp) rings as spectator ligands (groups that do not detach from the metal during the catalytic process). Systems that include two Cp rings are generally referred to as “bis-Cp” metallocenes, and systems that include a single Cp ring are referred to as “mono-Cp” metallocenes or “half” metallocenes. Cp-containing systems often require co-catalysts for their activation, such as alumoxanes (e.g., methyl aluminoxane) or various boron-based activators (often combined with aluminum based quenchers or scavengers). When alumoxane is employed as co-catalyst, a usually taken in large excess relative to the pre-catalyst—the common range being 1000-10000:1 MAO:pre-catalyst.
A specific class of half metallocenes termed “constrained geometry catalysts” was found to incorporate high-olefins readily and has found commercial applications in copolymerization of ethylene and high-olefins to produce linear low density polyethylene (LLDPE).
Metallocenes have been investigated very intensively during the past three decades and numerous scientific articles and patents describing various structural modifications and their applications in propylene and other olefin polymerizations have appeared. Correlations between the symmetries of the catalysts and the tacticities of the resulting polymers were established (Ewen's Rules). Of the metallocenes, some of the most studied were the zirconocenes. Yet the commercial applications of the metallocenes are limited: the price of the successful metallocenes is high relative to the Ziegler-Natta heterogeneous catalysts, and the isotacticity degree of the resulting polypropylene is often inferior.
In the past 15 years, there has been interest in development of “cyclopentadienyl-free systems”, e.g., pre-catalysts devoid of a cyclopentadienyl ring. Such modified catalysts are hoped to lead to polymers of new or improved properties. These non-metallocene systems include all kinds of transition metals, and still, the most promising systems in terms of activities and stereospecificities are tend to be based on the group 4 metals. Some of these catalysts have shown remarkable activities including living polymerization of high olefins at room temperature, highly active polymerization of ethylene, and the combination of living and isospecific polymerization of high olefins. Yet, except for scarce cases, the tacticity induction in propylene polymerization by non-metallocenes is inferior in comparison to the best metallocenes and to the latest generation heterogeneous Ziegler-Natta catalysts.
Salans are sequential tetradentate dianionic {ONNO}-type ligands that include two neutral amine-type N-donors and two anionic phenolate-type O-donors. Group 4 complexes of Salan ligands that exhibited isospecific polymerization of 1-hexene were first introduced by one of the present inventors (see, Kol et al., J. Am. Chem. Soc. 2000, 122, 10706-10707; and U.S. Pat. Nos. 6,632,899 and 6,686,490).
Group 4 complexes of Salan ligands as highly active catalysts in alpha-olefin polymerization were also disclosed in WO 03/091292. According to the teachings of WO 03/091292, propylene polymerization using the disclosed catalysts yielded a viscous liquid or a sticky solid of low molecular weight, rather than isotactic polypropylene of high molecular weight.
WO 2009/027516 discloses block-copolymers as compatibilizers that were prepared by previously published Salan complexes.
Since the year of 2,000, numerous scientific papers pertaining to group 4 catalysts based on Salan ligands and their applications in different types of polymerizations and copolymerizations of alpha-olefins and dienes, have been published.
However, none of the Salan catalysts described thus far was able to lead to practical catalysts for commercially relevant applications, such as highly active isospecific polymerization of propylene to yield high molecular weight polypropylene with a high melting point. Thus, none of the Salan catalysts was found to act in propylene polymerization as both a highly active catalyst that produces polymers with high tacticities.
For example, a zirconium catalyst of a symmetric Salan ligand that features a 1-adamantyl group in the ortho positions of both phenolate rings led to polypropylene with an isotacticity degree [mmmm] of 98.5% and melting point of 152° C. However, the activity of this catalyst was low: 4.8 grams polypropylene×mmol−1×[C3H6]−1×h−1 (Busico et al. Proc. Nat. Acad. Sci. 2006, 103, 15321).
A hafnium analogue of this catalyst led to higher tacticities, however, its activity was more than 10 times lower, so the molecular weight of the polymer obtained after 20 hours was only 7,200 gram/mol, corresponding to less than 200 repeat units, which is too low to give a meaningful melting transition (Cipullo et al. Macromolecules 2009, 42, 3869).
Zirconium catalysts of Salan ligands bearing halo-substituents (chloro or bromo) led to highly isotactic poly(vinylcyclohexane) (Segal et al. Macromolecules 2008, 41, 1612). However, the degree of isotacticity was reduced drastically when a “less bulky” monomer like 1-hexene was polymerized (Segal et al. Organometallics 2006, 24, 200).
A titanium catalyst of a symmetric Salan ligand that features the electron withdrawing iodo groups in the ortho, para positions of both phenolate rings showed a much higher activity of up to 390 grams polypropylene×mmol−1×[C3H6]−1×h−1, and a high molecular weight of Mn=240,000. However, its highest tacticity was [mmmm]=83% and correspondingly, the melting point was only 123° C. (Cohen et al. Macromolecules 2010, 43, 1689).
Attempts to produce C1-symmetric catalysts by devising Salan ligands that include a phenolate ring with stereo-directing bulky groups and a phenolate ring including activity-enhancing electron-withdrawing groups led to average values of the activities of the catalysts and average values of the tacticities of the resulting polymers (Cohen et al. Organometallics 2009, 28, 1391).
In addition, metal complexes of chiral Salan ligands were employed as catalysts for other transformations such as asymmetric catalysis. These include Salan ligands in which both of the amine-type N-donors are secondary amines (namely, each has one hydrogen substituent).
Strianese et al., Macromol. Chem. Phys. 2008, 209, 585-592; and Lamberti et al., Coord. Chem. Rev. 2009, 253, 2082-2097 teach Salan ligands assembled around dinaphthyl-diamine skeleton and bearing hydrogen atoms on the N-donors, and further teach complexes of these ligands. The complexes possess a C2-symmetry and give low activity and practically atactic polypropylene.
Matsumoto et al., Angew. Chem. Int. Ed. 2009, 48, 7432-7435 teach a Salan ligand which is based on aminomethylpyrrolidine, for use in asymmetric catalysis. Other publications by the same research group [Sawada et al., Angew. Chem. Int. Ed. 2006, 45, 3478-3480; Matsumoto et al., Chem. Commun. 2007, 3619-3627; Egami et al., J. Am. Chem. Soc. 2007, 129, 8940-8941; Matsumoto et al., Pure Appl. Chem. 2008, 80, 1071-1077; Egami et al., Angew. Chem. Int. Ed. 2008, 47, 5171-5174; Kondo et al., Angew. Chem. Int. Ed. 2008, 47, 10195-10198; Egami et al., J. Am. Chem. Soc. 2010, 132, 5886-5895; and Egami et al., J. Am. Chem. Soc. 2010, 132, 13633-13635] teach Salan ligands built around chiral diamine skeletons, in which the substituents on both of the N-donors are hydrogen atoms. The ligands have a symmetry element and the preferred ligand wrapping mode around a group 4 element is fac-fac. All complexes were employed for asymmetric catalysis.
Manna et al., Dalton Trans. 2010, 39, 1182-1184 teach complexes of chiral Salan ligands featuring hydrogen substituents on the N-donors, and fac-fac wrapping mode around a group 4 element. These complexes were reported to exhibit cytotoxicity.
Additional references of interest include WO 2011/158241 and Meker et al., Dalton Trans 2011, 40, 9802.